Electroceramic component comprising inner electrodes

ABSTRACT

A ceramic component is proposed, whose ceramic base body consists of a sequence of ceramic layers. Contact surfaces are arranged on the outer surface of the ceramic base body and are connected with internal electrodes in an electrically conductive manner. In order to obtain increased pulse resistance of the component, at least two internal electrodes are arranged on each ceramic layer, wherein only internal electrodes that are arranged on adjacent ceramic layers overlap, and, at the same time, are connected with different contact surfaces of the ceramic base body.

Examples of electrical components having a ceramic base body andinternal electrodes are thermistors and varistors. The base body ofvaristors is frequently produced from a mixture of different metaloxides, for example, from zinc oxide. Varistors demonstrate anon-linear, voltage-dependent change in resistance, which is used toprotect an electronic circuit against over-voltage. In this process, theresistance value of varistors decreases with an increasing voltage thatis applied.

In the diversion of electrical over-voltage pulses by the varistor, theenergy of the pulse must be absorbed by the ceramic base body.Particularly in the case of short voltage pulses, strong local heatingof the ceramic base body occurs between the internal electrodes. In thisprocess, temperature equalization with the environment is not possible.This results in great temperature differences, which bring aboutmechanical stresses in the ceramic base body. In this process, theinternal electrode causes further weakening of the cross-sectional areaof the ceramic base body. For this reason, cracks can occur in theceramic body, in case of severe local heating of the varistor, and thesecan result in ceramic parts coming loose from the base body.

Conventional varistors have two contact surfaces on the faces of theirbase bodies. Internal electrodes are arranged in the interior of thebase body, which, in cross-section, are pushed into one another inchamber-like manner through all levels, wherein the internal electrodesare each alternately connected with a contact surface of differentpolarity (see FIG. 1A). A cross-section through the base body of thevaristor along a ceramic layer shows that a major portion of the surfaceof this layer is taken up by the internal electrode (see FIG. 1 b). Thisresults in weakening of the ceramic body in this region, so that in caseof a large electrical pulse, crack formation must be expected in theceramic base body, due to over-heating.

A different variant of varistors is known from the publication DE 199 31056 A1, in which internal electrodes are arranged in such a manner thattwo electrode stacks that do not overlap with one another are formed, ineach instance, each of which contacts a contact surface (see FIG. 2A). Across-section through the ceramic base body along a ceramic layer showsthat two internal electrodes 15A, 15B, which lie opposite one another,are arranged on a ceramic layer (see FIG. 2B). In the case of a largeelectrical pulse, local over-heating occurs also in the case ofvaristors having this construction, and this can result in fracture ofthe base body.

An NTC thermistor element is known from the published patent applicationDE 198 06 296 A1, which has at least one ceramic layer on which a longerand a shorter internal electrode are arranged, the ends of which areopposite one another, spaced apart. In this arrangement, the internalelectrodes can overlap, if they are connected with the same contactsurface arranged on the outer surface of the component, or also internalelectrodes can overlap if they are connected with different contactsurfaces. In the case of this thermistor element, as well, crackformation in the ceramic base body can be expected in case of a largeelectrical pulse.

It is therefore the object of the present invention to provide anelectrical component having a ceramic base body and internal electrodes,which can absorb an electrical pulse well, and, at the same time,demonstrates increased mechanical stability as compared with the stateof the art.

This object is achieved, according to the invention, via a componentaccording to claim 1. Advantageous embodiments of the component are theobject of dependent claims.

The invention relates to an electrical component having a ceramic basebody, which comprises a sequence of ceramic layers, as well as severalinternal electrodes arranged on the ceramic layers, where at least twointernal electrodes are arranged on a first ceramic layer and form afirst electrode level. At least two additional internal electrodes arearranged on a second ceramic layer, and form a second electrode level.Furthermore, two contact surfaces are arranged on the outer surface ofthe ceramic base body, which are connected with the internal electrodesin an electrically conductive manner. In this arrangement, only internalelectrodes that are connected with a different contact surface, in eachinstance, and are, at the same time, arranged on adjacent first andsecond ceramic layers, overlap.

A component according to the invention demonstrates an increased pulseresistance as compared with a conventional component. This isparticularly attributable to two characteristics of the componentaccording to the invention. For one thing, internal electrodes that areconnected with different contact surfaces, in other words, thatdemonstrate different polarity, overlap. This ensures that the energy ofan electrical pulse can be absorbed on a large area in the interior ofthe component. Furthermore, a component according to the invention hasat least two internal electrodes on a ceramic layer, which has theresult that the cross-sectional area of the ceramic base body,corresponding to the surface of a ceramic layer not covered by internalelectrodes, is significantly greater than in the case of theaforementioned conventional components. This has the result that theceramic base body demonstrates increased mechanical stability and,therefore, also increased pulse resistance.

In the case of a variant of the component according to the invention,the contact surfaces are arranged on opposite faces of the ceramic basebody. In this arrangement, the internal electrodes arranged on the firstceramic layer are connected with the same contact surface, in eachinstance. This means that the internal electrodes are connected with theother contact surface on the adjacent second ceramic layer, in eachinstance. In this construction of the component according to theinvention, internal electrodes arranged on different ceramic layerstherefore overlap, in each instance, wherein the internal electrodes onone layer are connected with the same contact layer, in each instance,and the overlapping internal electrodes of the adjacent layer areconnected with the other contact surface, in an electrically conductivemanner (see FIGS. 3A and 3B).

In another variant of the invention, the contact surfaces are alsoarranged on opposite faces of the ceramic base body, where the internalelectrodes are, however, connected with different contact surfaces, ineach instance. These internal electrodes overlap with internalelectrodes on the adjacent second ceramic layer, in each instance, whichare connected with the other contact surface, in each instance (seeFIGS. 4A and 4B).

In another embodiment of the invention, the ceramic base body can be analternating arrangement of several first and second ceramic layerregions. In this way, it is possible to construct large ceramic basebodies having a plurality of overlapping internal electrodes havingdifferent polarity.

In another advantageous embodiment of the invention, the main axes ofoverlapping internal electrodes are made to maximally cover one another,so that the internal electrodes overlap maximally. In this arrangement,the main axes indicate the preferred direction of the greatest expanseof an internal electrode. The internal electrodes are advantageouslyconfigured as electrode surfaces, in order to produce the greatestpossible overlap area.

The ceramic base body of the component according to the inventionadvantageously comprises a varistor ceramic. In this case, theelectrical properties, in other words, the varistor effect, aredetermined by those ceramic volume regions of the component that arearranged between overlapping internal electrodes having differentpolarity. Since, in the case of the component according to theinvention, a plurality of internal electrodes can be overlapped, it is,as a result, possible to implement a varistor having a large proportionof ceramic volume regions that lie between overlapping internalelectrodes. Such components demonstrate an increased capacitance andpulse resistance.

In the following, the invention will be explained in greater detail,using exemplary embodiments and figures.

FIGS. 1A and 1B show a cross-section and a longitudinal section througha conventional component having overlapping internal electrodes.

FIGS. 2A and 2B show a cross-section and a longitudinal section througha component according to the state of the art, having non-overlappinginternal electrodes.

FIGS. 3A and 3B show a perspective view of and a longitudinal sectionthrough a variant of a component according to the invention.

FIGS. 4A and 4B show a perspective view of and a longitudinal sectionthrough another variant of the component according to the invention.

FIG. 1A shows a cross-section through a conventional component having acomponent base body 1. It can be seen that internal electrodes 15, whichare arranged on adjacent ceramic layers 5 and 10, overlap. The internalelectrodes contact different contact surfaces 20A and 20B, in eachinstance. FIG. 1B shows a longitudinal section through the ceramic layer10 referenced as I in FIG. 1A. It can be seen that in this ceramic layer10, the cross-section of the ceramic layer is very greatly reduced, andtherefore weakened, by means of the internal electrode 15. Such acomponent therefore demonstrates reduced mechanical stability, whichalso has a negative effect on the pulse resistance of the component.

FIG. 2A shows a cross-section through a conventional component, in whichtwo internal electrodes are present per ceramic layer 5 and 10,respectively. These internal electrodes are an integral part ofelectrode bundles 25A and 25B, in each instance, which bundles contact adifferent contact surface 20A and 20B, in each instance, and standopposite one another in the component, without overlapping. Because ofthe lack of overlap of the electrode bundles, insufficient portions ofceramic volume regions that lie between internal electrodes havingdifferent polarity are present in a component having this construction,in order to be able to absorb high-energy electrical pulses. FIG. 2Bshows a longitudinal section through the ceramic layer 10 that isreferenced as I in FIG. 2A. It can be seen that two internal electrodes15A and 15B stand opposite one another and contact different contactsurfaces 20A and 20B, in each instance.

FIG. 3A shows a perspective view of a first variant of the componentaccording to the invention. It can be seen that two internal electrodes15A and 15B are present, in each instance, on two adjacent ceramiclayers 5 and 10, respectively, wherein the two internal electrodes thatare arranged on one ceramic layer contact the same contact surface. Theyoverlap with internal electrodes arranged on adjacent ceramic layers,which contact different contact surfaces 20A and 20B, in each instance,as is indicated with double arrows in FIG. 3A. Because of the overlap ofthe internal electrodes having different polarity, a particularly largearea is created to absorb the electrical pulses. FIG. 3B shows alongitudinal section through the ceramic layer region 5 referenced as Iin FIG. 3A. It can be seen that two internal electrodes 15A and 15B arearranged on this ceramic layer 5, which contact the same contact surface20B, in each instance. Because of the two internal electrodes on theceramic layer, an E-shaped cross-section of the ceramic base body occurshere. In comparison with conventional components (see FIG. 1B, forexample), this results in a greater cross-sectional area, and thereby inincreased mechanical stability of the component base body.

FIG. 4A shows a perspective view of another variant of the componentaccording to the invention. It can be seen that two internal electrodesare arranged, per ceramic layer, on adjacent ceramic layers 5 and 10,respectively, wherein those internal electrode that are arranged ondifferent ceramic layers and demonstrate different polarity, in otherwords, contact different contact surfaces, overlap. The overlap isschematically shown by the double arrows in FIG. 4A. FIG. 4B shows alongitudinal section through the ceramic layer 5 referenced as I in FIG.4A. It can be seen that two internal electrodes 15A and 15B, whichcontact different contact surfaces 20A and 20B, in each instance, arearranged on this layer. As a consequence of this, an S-shapedcross-section of the ceramic layer 5 occurs in this region. In the caseof this variant, as well, the cross-sectional area of the ceramic basebody is greater in each ceramic layer, while, at the same time, theinternal electrodes overlap, than in the case of conventionalcomponents, so that increased mechanical stability and thereby alsoincreased pulse resistance are achieved.

The components according to the invention can be produced, in aparticularly advantageous manner, via multi-layer technology. In thisarrangement, the ceramic base body is produced by stacking ceramic greenfilms on one another. In this case, the internal electrodes can beapplied by printing on the ceramic green films. In this process, atleast two internal electrodes are present per ceramic layer, accordingto the invention, which are separated from one another by an unprintedregion, so that they do not overlap, and a large cross-section of theceramic body is achieved in this region.

Varistors according to the invention, having a varistor voltage of 22 to24 volts, were subjected to electrical pulses of up to 50 joules. Inthis arrangement, test measurements showed that the varistor voltage inthe ceramic base body does not change, which is a clear indication thatthe base body was not damaged by the electrical pulses.

The invention is not limited to the exemplary embodiments listed here.Other variations of the component according to the invention arepossible, particularly with regard to the number of internal electrodesper ceramic layer.

1. An electrical component comprising: a ceramic base comprising ceramiclayers; electrode levels between the ceramic layers, where eachelectrode levels comprises at least two electrodes; and contact surfaceson an outer surface of the ceramic base, the contact surfaces beingelectrically connected to the electrodes; wherein electrodes overlapthat are on different electrode levels and that are connected todifferent contact surfaces.
 2. The electrical component of claim 1,wherein the contact surfaces are on opposite faces of the ceramic base;and wherein all electrodes on an electrode level are connected to a samecontact surface.
 3. The electrical component of claim 1, wherein thecontact surfaces are on substantially opposite faces of the ceramicbase; and wherein electrodes on an electrode level are connected todifferent contact surfaces.
 4. The electrical component of claim 1,wherein the electrode levels comprise first and second electrode levelsthat alternate in the ceramic base, the first electrode levelscomprising first electrodes that connect to a first contact surface andthe second electrode levels comprising second electrodes that connect toa second contact surface.
 5. The electrical component of claim 1,wherein electrodes that overlap each have areas and a majority of theareas overlap.
 6. The electrical component of claim 1, wherein theelectrodes are substantially flat.
 7. The electrical component of claim1, wherein the ceramic base comprises a varistor ceramic.
 8. Theelectrical component of claim 1, wherein the electrode levels comprisefirst and second electrode levels that alternate in the ceramic base,the first electrode level comprising a first electrode that connects toa first contact surface and a second electrode that connects to a secondcontact surface, the second electrode level comprising a third electrodethat connects to the first contact surface and a fourth electrode thatconnects to the second contact surface, the first and fourth electrodesoverlapping and the second and third electrodes overlapping.
 9. Theelectrical component of claim 8, wherein the electrodes aresubstantially flat.
 10. An electrical component comprising: a ceramicbase; first and second contact surfaces on exteriors of the ceramicbase; and electrodes internal to the ceramic base, the electrodes beingarranged on levels in the ceramic base, the electrodes comprising: afirst electrode on a first level connected to the first contact surface;a second electrode on the first level connected to the first contactsurface; a third electrode on a second level connected to the secondcontact surface, at least part of the third electrode being below thefirst electrode; and a fourth electrode on the second level connected tothe second contact surface, at least part of the fourth electrode beingbelow the second electrode.
 11. The electrical component of claim 10,wherein the first and second contact surfaces are on substantiallyopposite sides of the ceramic base.
 12. The electrical component ofclaim 10, wherein the electrodes are substantially flat.
 13. Theelectrical component of claim 10, wherein the first and third electrodehave different polarities and the second and fourth electrodes havedifferent polarities.
 14. The electrical component of claim 10, whereinthe ceramic base comprises stacked green films.
 15. An electricalcomponent comprising: a ceramic base; first and second contact surfaceson exteriors of the ceramic base; and electrodes internal to the ceramicbase, the electrodes being arranged on levels in the ceramic base, theelectrodes comprising: a first electrode on a first level connected tothe first contact surface; a second electrode on the first levelconnected to the second contact surface; a third electrode on a secondlevel connected to the second contact surface, at least part of thethird electrode being below the first electrode; and a fourth electrodeon the second level connected to the first contact surface, at leastpart of the fourth electrode being below the second electrode.
 16. Theelectrical component of claim 15, wherein the first and second contactsurfaces are on substantially opposite sides of the ceramic base. 17.The electrical component of claim 15, wherein the electrodes aresubstantially flat.
 18. The electrical component of claim 15, whereinthe first and third electrode have different polarities and the secondand fourth electrodes have different polarities.
 19. The electricalcomponent of claim 15, wherein the ceramic base comprises stacked greenfilms.
 20. The electrical component of claim 15, wherein the ceramicbase comprises a varistor ceramic.